1. Field of the Invention
The present invention relates generally to retroreflectors, and relates more particularly to retroreflectors with variable reflective surface shapes.
2. Description of Related Art
Retroreflectors are used in a number of applications, such as in optical communications. Retroreflectors are useful for low power communications because of their ability to modulate an incoming light signal, for example, and return a modulated signal directly to the original source. A number of types of retroreflectors are known, including corner-cube retroreflectors, cat's eye retroreflectors, single mirror retroreflectors and lens and parabola retroreflectors. In general, retroreflectors have the property of taking in an incident beam, received from a source such as an optical or RF beam, and directing the beam back to the source through one or more reflections. Retroreflectors provide the advantage of directing a beam carrying information back to a source without the need for a transmitter at the retroreflection site. Retroreflectors can also be arranged in an array to provide a larger target, multiple channels, or other advantages.
One technique used to transmit information over a retroreflected beam is to amplitude modulate the reflected beam, or provide a multiple amplitude coding for the beam, such as is suitable for use in binary encoded communications systems. The incident beam in such a scenario is permitted to reflect back to the source or not depending on a modulation of the retroreflector device. Modulation of the retroreflective device is typically achieved by varying the reflective path in the retroreflector. For example, a reflective surface in the retroreflector is tilted to change the direction of a reflected beam, so that the reflected beam is no longer returned to the beam source.
A number of different modulation techniques are known for interrupting the retroreflective activity of a retroreflector. One type of modulator that involves mechanical displacement of a reflective surface to redirect the reflective beam uses micro-electromechanical system (MEMS) to actuate a mechanical system that causes tilting of a reflective surface to redirect the reflected beam. MEMS technology is useful for low energy, small displacement mechanical activity. Using MEMS technology, a micromechanical reflective surface can be tilted based on a modulating signal to modulate the reflection of the incoming beam to provide amplitude modulation or binary encoding of the returned beam. One drawback of this type of retroreflector arrangement is that a relatively large angular displacement is used to cause the retroreflector to reflect or not reflect the incoming beam. The relatively large angular displacement represents challenges with respect to the device geometry, energy consumed by the device, operation frequency, and other performance criteria. For example, the modulation bandwidth may be limited by the response time of the tilted reflective surface, the limitation being exacerbated by inherent response time delays of the modulation device, such as may be observed with voltage slew notes.
Other types of MEMS based retroreflective devices have been developed that can limit the tilt angle of the reflective surface used to obtain an on/off modulation of a reflective beam to a source. Such a retroreflective device is illustrated in U.S. application Ser. No. 10/661,028, now U.S. Pat. No. 7,729,030 where a reflective surface is displaced through an angular range that permits the device to switch between a reflective and transmissive mode of operation. However, the device adds additional components that complicate the structure and consume a fair amount of energy by tilting the reflective surface through an angular displacement with a MEMS type device.
Other types of retroreflective devices that can be modulated include Fabry-Perot devices that operate to transmit or reflect light depending on a configuration of parallel plates arranged at an angle to the incident light beam. However, such a device requires two or more reflective surfaces in conjunction with a retroreflector. Such a configuration can be somewhat complex and have a limited bandwidth due to device response time.
Another type of modulated retroreflector uses an optical shutter across an aperture of a normal retroreflector to permit or prevent transmission of an incident beam or retroreflected light. This type of modulated retroreflector can be somewhat expensive and represents other design challenges due to the absorption of energy when the shutter is in a non-transmissive state. As with other modulated retroreflectors described above, the optical shutter device also can have a limited bandwidth due to device component response times.
It would be desirable to obtain a retroreflector device that consumes less energy than known devices on a scale operable with MEMS type structures.